1988 Speed Zone Surveyu
A , Automobile Club of Southern California
HEAOQUAAIEAS. 2601 SOUTH F I G U E A 0 A S I A E E I • LOS ANGELES, CALIFORNIA 90007329/
VIA ILIN G: P.0. AOI 2190 TERMINAL ANMEI • LOS ANGELES, CALIFORNIA 90051 0690
MKHWAY ENGINEERING DEPARTMENT
A RUTH GRIM, MANAGER
May 19, 1988
Mr. Frank Reynolds
Director of Public Works/
City Engineer
City of La Quinta
P.O. Box 1504
La Quinta, CA 92253
Dear Mr. Reynolds:
In response to your earlier request, we are pleased to submit the
attached report on speed zones for the City of La Quinta.
We wish to express our appreciation for the cooperation and assist-
ance extended us by Roger Hirdler and Officer Darrell Czajkowski
whose insights were important to the success of the study.
If you have any questions about any of the recommendations contained
in the report, please give us a call.
Cordially,
�i/ox*enGary
Tr is Engineer
bk
a
SPEED ZONE SURVEY
FOR THE
CITY OF LA QUINTA
May 1988
BY THE
Highway Engineering Department
OF THE
Automobile Club of Southern California
AVe. 52, Bermudas, Tampico, Eisenhower, Jefferson and Washington
SPEED ZONE SURVEY
FOR THE
CITY OF LA QUINTA
INTRODUCTION
a
The Highway Engineering Department -of the Automobile Club of Southern
California submits this report in response to a request from the
City of La Quinta for assistance in the review of speed zones on 6
City streets described below. The purpose of the study was to deter-
mine appropriate speed zones for these streets consistent with the
requirements of California Vehicle Code Section 40802(b).
Simply stated, the Vehicle Code requires that posted speed limits
on the more important streets of a community be based upon a rational
evaluation of roadway and roadside conditions, a review of the acci-
dent history and measurements of prevailing speeds before radar can
be used for their enforcement.
STREETS SURVEYED
The streets surveyed and limits of the study are shown on a map
which is included in the "APPENDIX" section of this report and are
described as follows:
Avenue 52 - Avenida Bermudas to Adams Ball Field
Avenida Bermudas - Calle Durango to Calle Madrid
Calle Tampico - Washington Street to Eisenhower Drive
Eisenhower Drive - Washington Street to Avenue 50
Eisenhower Drive - Calle Tampico to Avenida Bermudas
Jefferson Street - Highway 111 to Westward Ho Drive
Washington Street - Highway 111 to 3400 feet south
STUDY PROCEDURE
Nationally recognized study procedures, consistent with CVC Section
627 and the methods described in Chapter 8 of the California State
Traffic Manual were followed in this study.
Speed data were collected by City staff following a training exer-
cise conducted by Auto Club representatives covering recognized pro-
cedures. They recommend using a radar device in an unmarked car with
the results being recorded on "Motor Vehicle Speed" forms (see
"APPENDIX"). The data were fed into an Auto Club computer which pro-
duced a printout of the various elements of speed discussed in the
"ESTABLISHING SPEED ZONES" section which follows. A computer run
was made for each survey location and those printouts are bound with
the appropriate "Motor Vehicle Speed" forms in the "APPENDIX".
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Acaitlonally, tre file of acciaents containEd fn the Statewide
Integrated Traffic Records System was analyzed by the Auto Club for
calendar years 1986 and 1987. Generally, accidents related to
excessive speed were not a problem of significance but where excep-
tions were noted, the speed related accident history was considered
as an influence upon recommendations for posted speed limits.
As a final element of the study, each street was driven in the
presence of City staff to identify existing roadway and roadside
conditions, and those observations in combination with the analysis
of accidents, formed the basis for the improvement suggestions
included in the "RECOMMENDATIONS" section of this report.
ESTABLISHING SPEED ZONES
The purpose of speed zoning is to establish realistic posted limits
which aim to:
• Encourage voluntary compliance by a majority of
drivers.
• Provide an enforcement tool that effectively
separates the occasional violator from the reason-
able majority.
* Facilitate safe and orderly traffic flow .
Two measurements extracted from the speed data are key to the estab-
lishment of reasonable speed limits --the Prevailing speed and the
Pace speed. The Prevailing speed is also known as the 85th percen-
tile or critical speed.
The Prevailing speed is that speed at or below which 85 percent of
the observed vehicles were travelling. It is usually within 2 mph
of the upper limit of the Pace.
The Pace speed is the 10 mph range of speed which contains the
greatest number of observed vehicles.
Unless accident statistics or unexpected roadway conditions dictate
otherwise, speed limits should be set at or near the Prevailing speed.
The posted zone will normally coincide with the 5 mph increment next
below the Prevailing speed (e.g. Prevailing speed = 38 mph, speed
limit posting = 35 mph).
A more detailed explanation of speed zoning principles and methods
is contained in the "Speed Zoning, Why and How" booklet submitted
with this report.
DATA PRESENTATION
Table 1 summarizes the speed data and shows the effects upon traffic
of the recommended limits. The percent of traffic violating each
existing speed limit is also noted. A high violation rate would
indicate an unreasonable and not effectively enforceable speed limit,
thereby leading to unjustifiable tolerances of 15 mph or more before
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enforcement action is applied. Cnanging to realistic speed IimiiitS
would result in better driver compliance and would more clearly
identify that segment of the driving population which legitimately
deserves enforcement action without excessive tolerances.
The view held by many, is that a 5 mph tolerance for speedometer and
radar error is the most that can be justified. The Automobile Club
of Southern California generally supports that enforcement philosophy,
providing that speed zones are established according to prescribed
practices. This is particularly true of zones which are posted at
or within 2 mph of the prevailing speed where violations of 5 mph
or more are so clearly out of line with the behavior of the average
driver as to constitute a significant risk.
Table 1 also includes a summary of the 2 year total number of acci-
dents which have occurred on each of the roadways and, of the total,
the number of collisions which were related to excessive speeds.
Table 2 presents a more detailed look at accidents including their
severity and the involvement of alcohol as well as the information
mentioned above. The summary of accidents can be helpful in establish-
ing enforcement and safety improvement priorities.
RECOMMENDATIONS
AVENUE 52 - Post a 50 mph speed limit between Adams Ball field
and the curve west of Washington Street. The portion south
and west of subject curve should be surveyed after road
construction activities are completed and posted accordingly.
• Place edge stripes throughout the entire section
of road to more clearly define the travelled way.
• Review the chevron alignment sign installations
for both directions at the"S" curve east of
Washington St. and the 900 curve west of Washing-
ton Street. Three signs should be in the
motorist's view as they travel through the curve
(see page 4-34 of State Traffic Manual).
AVENIDA BERMUDAS - Retain 45 mph speed limit posting.
• Remove shrubbery within the right-of-way at
Calle Sonora, Avenida Nogales and Avenida Chillon
to improve visibility.
• Review the chevron alignment sign installation
at the curve south of Avenida Arroba to ensure
compliance with the "3 signs in view" require-
ment of the State Traffic Manual.
• Stripe a double yellow centerline through the
short tangent between the curves at Calle
Madrid and Calle Chillon.
U
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CALLE 7hN,FiCO - post a 45 mp- speed ilt between, h&S,n!ny_.
Street and Desert Club Drive and 35 mph between Desert
Club Drive and Eisenhower Drive.
• Install more prominent signs and markings to
define the right -turn only lane into the new
subdivision on the north side east of Avenida
Bermudas.
• Review the need for extending the double
yellow centerline striping to the east from
the Avenida Bermudas curve based upon passing
sight distance requirements.
EISENHOWER DRIVE - Retain 45 mph between Avenida Bermudas and Calle
Durango, and between Avenue 50 and Washington Street.
The speed data on the section between Avenue 50 and Washington
Street would normally call for a posted limit of 50 mph and
the majority of the roadway appears able to safely accommodate
such speeds. However, there is a rather severe visibility
restriction at the intersection of Eisenhower Drive and
Coachella Drive, caused by a combination of factors (road
curvature, narrow parkway and presence of a block wall), which
cannot support a 50 mph approach speed. For this reason and
because the survey section is not long enough for multiple
speed zones, a 45 mph zone is recommended.
The City has already taken steps to cope with the visibility
restriction by modifying the roadway striping and delineation
for eastbound traffic and by installing an advance intersec-
tion warning sign with a flashing beacon. That has helped,
but further improvements are desirable.
Reportedly, discussions are underway for construction of a
traffic signal at Coachella Drive and its completion should
resolve the issue satisfactorily allowing, then, for a posted
limit more in line with driver expectancy.
In the interim, it is suggested that consideration be given
to relocating the stop line on Coachella Drive 6-7 feet ahead
of its present position in order to improve visibility to the
maximum extent possible.
Additional recommendations for this roadway are as follows:
• Install "COACHELLA DRIVE" street name signs in
advance of the intersection for both directions
of travel.
• Place appropriate arrow pavement markings to
supplement the lane assignment signs controlling
the 3 eastbound turn lanes at Washington Street.
L
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• Eliminate the rignt-turn cr:;j restri t��rs -.��s�c
upon the westbound right lane at the first two
intersections west of Washington Street. There
is simply not enough right -turn demand to justify
the restriction and the potential confusion it
creates.
Instead, allow the lane to be used by through
traffic and, supplemented by appropriate sign-
ing and arrow markings, terminate the lane
markings far enough away from the point where
the road narrows to allow adequate opportunity
for traffic to merge.
• Replace the existing "RIGHT TURN ONLY" sign
for southbound traffic at Avenue 50 with a
"RIGHT LANE MUST TURN RIGHT" sign positioned
for better visibility.
• Repaint the worn centerline markings between
Avenida Bermudas and Calle Tampico.
• Remove shrubbery within the right-of-way at
Calle Sonora, Calle Ensenada and at Calle
Temecula to improve visibility.
JEFFERSON STREET - Post 40 mph between Highway 111 and Westward
Ho Drive.
• Remove "CAUTION" warning signs for both directions
in advance of the marked golf cart crossing south
of Westward Ho Drive and install more explicit
W 54 pedestrian crossing symbol signs adjacent
to the crossing.
• Replace "END 35 ZONE" sign for northbound traf-
fic north of Westward Ho Drive with an R3
"END 40 MPH SPEED LIMIT" sign.
• Placean R2-4 "40 ZONE AHEAD" sign for south-
bound traffic north of Westward Ho Drive.
• Place "DIP" pavement markings along with W32
"DIP" warning signs and appropriate W6 advisory
speed plates for both directions of travel in
advance of the dip at the south side of Highway
111.
WASHINGTON STREET - Post 45 mph from Highway 111 to 3400 feet
south.
While the speed data for this section of highway would normally
support a posting of 50 mph, the roadside conditions and acci-
dent record justifies a 45 mph limit. These roadside condi-
tions include the presence of single family residences along
the west side of the road, all with direct access, and the
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need to zcck froi:. c"1veWajs c, -.-_o the nlyGwdy. ..,e c:.cident
history also justifies the slight speed zone deviation with
59% of the accidents primarily caused by excessive speed.
• With the help of Caltrans, immediately initiate
a detailed investigation of accidents and road
conditions at the Highway 111 intersection in
an attempt to identify improvements which will
help to reduce hazards at this accident prone
intersection. The Auto Club would welcome the
opportunity to participate in this study if
our help is desired.
• Install an R3 "END 45 SPEED LIMIT SIGN" for
southbound traffic at the end of the desig-
nated speed zone.
CONCLUSIONS
This report verifies or recommends establishment of speed zones that
benefit all road users in LA QUINTA.
The speed zones recommended will normally not increase operating
speeds but rather adjust the law on the most important streets in
the City to more closely reflect the reasonable actions of the
majority of motorists. A look at the speed data will prove this
point as most of the previously surveyed speed zones are still valid
and need not be changed.
City staff and law enforcement officers should continually review
traffic operations to:
• identify needed intersection and mid -block roadway improve-
ments,
• monitor accident trends,
e review speed trends and make appropriate adjustments in
speed limits and enforcement activity,
• pursue educational and training opportunities.
Finally, City Council action by resolution or ordinance will be
needed for the legal establishment of all speed zones not covered
by previous action.
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